2. Motivation and Background AR4 Chapter 10: In summary, all models show continued ENSO interannual variability in the future no matter what the change in average background conditions, but changes in ENSO interannual variability differ from model to model. Based on various assessments of the current multi-model archive, in which present-day El Niño events are now much better simulated than in the TAR, there is no consistent indication at this time of discernible future changes in ENSO amplitude or frequency.

3. Motivation and Background Largely a review of GCM studies Separate changes in mean (i.e. background) climate and ENSO variability Consider that there are errors in models and uncertainties in projections

4. Changes in Background Conditions

5. Robust Changes in the Hydrological Cycle in GCMs

9. © Crown copyright Met Office thermocline upwelling Normal Conditions

10. © Crown copyright Met Office Climate Change thermocline upwelling

11. thermocline upwelling © Crown copyright Met Office Climate Change (SST Anomalies) Not El Nino-like Trade winds weaken SSTs warm more on the equator than off The equatorial thermocline shoals and the stratification increase Upwelling weakens

12. © Crown copyright Met Office El Niño Conditions (SST Anomalies) thermocline upwelling

13. Changes in ENSO Variability

14. © Crown copyright Met Office Changes in ENSO variability 2050-2100 in CMIP3 A1B experiments van Oldenborgh et al 2005

15. thermocline upwelling © Crown copyright Met Office El Niño Features and Processes atmospheric damping SST response to thermocline anomalies SST response to wind stress anomalies upwelling surface zonal current zonal advective feedback external noise Walker Circulation

16. Balance of ENSO Processes in Each Model van Oldenborgh et al 2005

17. Process/FeedbackImpact on ENSO variability Mean upwelling and advectionup Thermocline feedbackup SST/wind stress (Ekman) feedbackup Surface zonal advective feedbackno change Atmospheric dampingdown Atmospheric variability? Other processes e.g. TIW? ENSO Variability: Processes and Feedbacks

18. Summary and Conclusions Background Conditions The tropical easterly trade winds are likely to weaken Models show SSTs warm more on the equator than off The equatorial thermocline shoals and the stratification of the thermocline increases Upwelling weakens Variability ENSO variability is controlled by a delicate balance of amplifying and damping feedbacks, and it is likely that one or more of the major physical processes that are responsible for determining the characteristics of ENSO will be modified by climate change While the possibility of large changes in ENSO cannot be ruled out, research conducted to date does not yet enable us to say precisely whether ENSO variability will be enhanced or damped, or if the frequency of events will change (because of errors in models)

20. East-Pacific vs Central Pacific/Modoki Lee and McPhaden 2010

21. Opportunities and Questions: I Finding statistically significant changes in ENSO under high levels of greenhouse gases is a mitigation-relevant scientific problem – what about ENSO characteristics in the next 10-30 years? New project; Tom Russon, Sandy Tudhope, Mat Collins, Gabi Hegerl, … Assess long-term natural variations in ENSO from coral archives and model simulations CMIP5 and Isotope-enabled HadCM3, palaeo-corals from the Galapagos from last 1000 years

22. Opportunities and Questions: II Are CMIP3 findings confirmed in CMIP5 models? Can we design and implement better metrics? We expect uncertainties, so how do we deal with them? Funding proposal; Mat Collins, Eric Guilyardi, Fei-Fei Jin, Axel Timmermann, Will Roberts, Geert Jan van Oldenborgh, … New CMIP5 models and simulations, new metrics Use ENSO models of intermediate complexity ENMICs to map out a much larger region of possible ENSO futures and constrain with observations/(re)analysis products

23. Opportunities and Questions: III Detection and attribution of observed changes (signal- to-noise issue) Deeper understanding of the role of the ocean especially clouds Reconciling past background and variability changes from palaeo-archives (Pliocene, mid-Holocene, LGM) with model simulations …

24. © Crown copyright Met Office thermocline upwelling El Niño Conditions

25. HadCM3 Perturbed Physics Ensemble 33 ensemble members Anthro and natural forcings SRES A1B Mean ENSO strength and frequency is sensitive to forcing in 20 th and 21 st centuries

26. Is El Niño Changing? Natural internal variability 5-95%tiles obs Ensemble mean change in NINO3 std. dev. in Hadley Centre models and observations Increase in ENSO variability over 20 th Century is seen in observations and model simulations forced by increasing greenhouse gases Future projections show larger, more frequent ENSO events Forced changes in ENSO variability 5-95%tiles Mat Collins, Met Office Hadley Centre

27. How does ENSO change? Switch from smaller El Nino events which propagate East- West… …to larger events which develop insitu or even propagate West-East More frequent events in the future

28. El Niño Southern Oscillation, Climate Models and Climate Change Climate models solve the dynamical equations of the atmosphere and ocean on a grid Because of restrictions in computer power, the grid is relatively coarse and sub-grid scale processes need to be parameterised Climate models are imperfect representations of the real world Nevertheless, the recent generation of climate models can simulate the basic physics and characteristics of ENSO

29. Understanding El Niño In Ocean–atmosphere General Circulation Models: Progress And Challenges Eric Guilyardi, Andrew Wittenberg, Alexey Fedorov, Mat Collins, Chunzai Wang, Antonietta Capotondi, Geert Jan Van Oldenborgh, And Tim Stockdale, BAMS 2009.